Silicon carbide (SiC) has a dielectric breakdown electric field that is larger than Si by 10 times or more and is capable of making a drift layer for maintaining withstand voltage to have a smaller thickness and a high concentration, and thus is a material capable of decreasing loss of an FET (field effect transistor). Accordingly, a MOSFET (metal oxide semiconductor FET) using SiC is receiving attention as a next-generation switching device with high withstand voltage and low loss.
As an example of a MOSFET, the operation principle thereof will be described, with reference to FIG. 2. In the figure, numeral 1 denotes an n+ substrate as a drain region, 2 denotes an n− drift layer, 3 denotes a p base region, 4 denotes a p+ contact region, 5 denotes an n+ source region, 6 denotes a gate dielectric film, 7 denotes a gate electrode, 8 denotes an interlayer dielectric film for electrically insulating the source and the gate, 9 denotes a source electrode, and 10 denotes a drain electrode. In the operation of the MOSFET, on application of a positive voltage on the gate electrode 7 in the state where a voltage is applied between the drain electrode 10 and the source electrode 9, an electron inversion layer is formed on the surface of the base region 3. As a result, an electric current flows from the drain electrode 10 to the source electrode 10 through the drain region 1, the drift layer 2 and the source region 5.
One of the factors that influence the capability of the device of the MOSFET is the quality of the interface between SiC and the gate dielectric film. The gate dielectric film is generally formed with silicon dioxide by such a method as a thermal oxidation method or a chemical vapor deposition (CVD) method, and the so-called MOS interface between silicon dioxide and silicon carbide has a large number of interface states (traps), which results in problems including considerable decrease of the channel mobility, increase of the on-resistance of the device, and increase of the loss on on-operation.
As a method for reducing the interface states and enhancing the mobility, for example, PTL 1 reports a method of performing a heat treatment with dinitrogen monoxide (N2O) gas after the thermal oxidation, or forming an oxide film by the nitriding treatment.
For example, PTL 2 reports a method of performing a direct heat treatment of SiC with dinitrogen monoxide (N2O) gas or nitrogen monoxide (NO), and then forming an oxide film by a deposition method.
For example, NPL 1 reports a method of performing implantation of nitrogen to a SiC substrate and then forming a gate dielectric film.
For example, PTL 3 reports a method of forming SiO2 to a thickness of from 0.3 to 0.9 nm and depositing thereon an aluminum oxide at a temperature of 300° C. or less to a thickness of from 10 to 100 nm, as a gate dielectric film on a SiC substrate.
As an example of using a high dielectric material film as a gate dielectric film, for example, PTL 4 reports a method of forming SiO2, depositing thereon a high dielectric material film, and depositing further thereon SiO2, as a gate dielectric film on a SiC substrate, thereby enhancing the dielectric breakdown characteristics.